VISUAL OEGANS OF VEEMES. 153 



on those appendages of tlie parapodia, wliicli are known as cirri, 

 as well as on tlie structures which, are formed from modifications 

 of these cirri (cf. § 106). The appendages just mentioned are pro- 

 vided with a large number of end-organs of sensitive nerves, and 

 thus become complicated tactile organs, which are of a somewhat 

 high grade, on account of their power of movement. 



A special complication of the tactile rods obtains in some Hiru- 

 dinea, where groups of these structures are embedded in the base of 

 cup- shaped organs. There is a large number of such organs in 

 the head, and they are scattered over the hinder rings of the body. 

 The arrangement of the sensory parts in depressions of the surface 

 of the body justifies us in supposing that we have here to do, not with 

 a special tactile organ, but with a sensory organ of general character. 



The tactile papillae are less differentiated than the tactile rods 

 or setae. They are developed in places where the body is covered by 

 a stronger cuticular layer, and are conical or wart-shaped elevations 

 of the cuticular layer, which are" traversed by a pore-canal. We 

 find these tactile papillfe in the Nematodes, where they are grouped 

 in a regular manner, some near the oi'al, and some round the genital 

 orifice. 



§ 123. 



Very little is certainly known as to their function, but organs 

 which may be regarded as sensory are formed by parts of the body 

 which either carry cilia, or have their epithelium distinguished by 

 some other peculiarity; such are the cephalic pits of many 

 Nemertina, and the similar parts in Polygordius. The clefts at 

 the side of the head lead into a narrow ciliated canal, which is con- 

 nected, either directly or by means of a fibrous chord, with the 

 cerebi'al ganglion. Perhaps the apparatus presented by the pro- 

 boscis of Balanoglossus may be regarded as an organ of this kind. 

 It is uncertain whether these organs serve for the perception of the 

 conditions of the surrounding medium, and possess a function 

 analogous to that of olfactory organs. 



Visual Organs. 



§ 124. 



The visual organs of the Vermes offer numerous examples of 

 the gradual evolution of an organ from an indifferent condition. In 

 many of the lower Vermes, Turbellaria, Trematoda, Nemertina, and 

 Rotatoria we often find, at the place where other forms have dis- 

 tinctly developed eyes, pigment -spots only, -which are arranged 

 symmetrically, and either placed directly on the brain, or close to 

 it. Nothing is known as to the mode of tei'mination of nerves in 

 these organs, so that it is uncertain whether such " eye-spots " 

 should be regarded as organs for the perception of light. 



